This invention relates to a method and apparatus for the fault time operation of an elevator or lift which is provided with a symmetrically arranged variable voltage, variable frequency (VVVF) electric power converting apparatus.
The fault time operation of an elevator utilizing a conventional symmetrically arranged VVVF power converting apparatus will be described with reference to FIG. 1, wherein the power of an induction motor 1 driving the elevator is supplied from an inverter 3 through an AC reactor 2. A tachometer generator 1a is coupled with the induction motor 1 for applying a voltage corresponding to the running speed of the motor to a control device 12 operable as a microcomputer and comprising a CPU 12a, a RAM 12b, a ROM 12c and an interface 12d. The control device 12 digitally controls switching signals applied to the bases of the transistors included in a converter 4 and the inverter 3. Since the operation of the microcomputer is well known in the art, a detailed description thereof is omitted.
The converter 4 is connected to a three-phase AC power source 8 through a contact 7a of a relay (not shown) and an AC reactor 6. A current transformer CT is connected to the converter input and its output is applied to the control device 12. The converter 4 converts AC power received from the source 8 into DC power, which is smoothed by a capacitor 5 and supplied to the inverter 3. The inverter converts the DC power back into AC power which is supplied through the reactor 2 to the induction motor as described above.
The three-phase AC voltage from the power source 8 is applied through a transformer 9 to a battery charger 10 including a diode bridge or the like which converts the AC voltage into a DC voltage for charging a battery 11. A serially connected circuit of the battery 11 and a contact 7b of the relay is connected in parallel with the capacitor 5, between input terminals A and B of the inverter 3. The AC voltage of the source 8 is also applied through another transformer 13 to the control device 12, which controls the conductance of component elements of the converter 4 and inverter 3 based on the output of the tachometer generator 1a and a command signal voltage Vp.
The relay contact 7a is spring biased open when its relay is deenergized, and vice versa for contact 7b.
The inverter 3 includes transistors and diodes that are connected with the transistors in parallel opposition. Under the control of the device 12, the inverter 3 is operated in a variable voltage, variable frequency mode by pulse width modulation. Since such operation is widely known, further description thereof is omitted.
The converter 4 also includes transistors and parallel opposition diodes. In the normal operation of the elevator wherein the relay contact 7a is closed and contact 7b is open, the induction motor 1 is energized from the three-phase AC power source 8 through contact 7a, reactor 6, the diodes in the converter 4, the transistors in the inverter 3, and the reactor 2.
In the regenerating mode, electric power is regenerated from the induction motor through the reactor 2, the inverter diodes, the converter transistors, reactor 6 and contact 7a to the power source 8.
The inverter 3 and the converter 4 are symmetrically constructed; their combination is termed a symmetrical VVVF apparatus.
A failure in the AC power source 8 is detected by the control device 12 from the output of the current transformer CT, in response to which the control device opens contact 7a and closes contact 7b to connect the charged battery 11 across terminals A and B. Inverter 3 then converts the DC battery power into AC power which is applied to the induction motor through reactor 2, so that the operation of the motor and elevator may continue. Although not indicated in the drawing, the control device 12 is also provided with an emergency power source similar to the battery 11.
In the conventional symmetrical VVVF apparatus as shown in FIG. 1, when the converter 4, inverter 3 and capacitor 5 subsystem becomes faulty for some reason so as to cause any one of the following phenomena:
(1) an abnormally large current flow through the transformer CT,
(2) the output of the tachometer generator 1a exceeding a predetermined value, or
(3) the difference between the command voltage Vp applied to the control device 12 and the output voltage of the tachometer generator becoming excessive (excessive acceleration),
the contact 7a is opened to interrupt the base currents of the transistors included in the inverter 3 and the converter 4, and a mechanical brake (not shown) is actuated to halt the movement of the elevator. To rescue persons from the stranded elevator to a nearby floor, contact 7b is closed a predetermined time after the brake actuation so that the converter 3 under application of the battery voltage is VVVF controlled to drive the induction motor as desired.
According to the above described emergency operation, however, if the malfunctioning component is the inverter 3, the induction motor cannot be operated by the inverter in a fault mode and the elevator passengers remain trapped at the braked position of the cage.